The present invention relates to a four-channel stereo broadcast receiver, and more particularly to a construction of a demodulator in such a receiver.
In place of the conventional two-channel stereo system, four-channel stereo systems have been developed to a point such that they are now practically available with respect to stereo disc records and stereo tape recorders. In the available systems, signals from four sound sources at the left front, left back, right front and right back positions (hereinafter referred to, respectively, as LF, LB, RF and RB for the sake of simplicity) are reproduced by four speakers disposed at the left front, left back, right front and right back positions for reproducing the original sounds with higher fidelity. Simultaneously therewith, in the field of FM-broadcasting, trials for four-channel stereo broadcasting are about to be conducted experimentally.
Although various systems are known to implement four-channel stereo broadcasting, as a representative one a system is known, in which a signal of (LF + LB) + (RF + RB) is borne by a main channel M, a signal of (LF + LB) - (RF + RB) is borne by a first sub-channel S.sub.1, a signal of (LF - LB) + (RF - RB) is borne by a second sub-channel S.sub.2, a signal of (LF - LB) - (RF - RB) is borne by a third sub-channel S.sub.3, and these signals are transmitted together with a pilot signal P (19 KH.sub.z) as a synthesized signal, as shown in FIGS. 1a - 1d. Then the first sub-channel S.sub.1 comprises a double side-band signal (DSB) obtained by modulating an m-multiple (normally double) frequency-multiplied signal (38 KH.sub.z) of the pilot signal with the signal of (LF + LB) - (RF + RB). The second sub-channel S.sub.2 comprises a double side-band signal (DSB) obtained by modulating an m-multiple frequency-multiplied and phase-shifted (normally by .pi./2) signal (38 KH.sub.z) of the pilot signal with the signal of (LF - LB) + (RF - RB). The third sub-channel S.sub.3 comprises a single side-band signal (SSB) obtained by modulating an n-multiple (m&lt;n and normally n=4) frequency-multiplied signal (76 KH.sub.z) of the pilot signal with the signal of (LF - LB) - (RF - RB) (In the systems shown in FIGS. 1a and 1c a lower side-band is selected, while in the systems shown in FIGS. 1b and 1d an upper side-band is selected.). Here it is to be noted that the third sub-channel may comprise, in some cases, an asymmetrical or vestigial side-band signal (VSB) that is one kind of single side-band, as shown in FIGS. 1c and 1d.
The present invention relates to a construction of a demodulator in a receiver to be used in the four-channel stereo broadcasting systems of the type described above. As the construction of such receiver, a circuit construction as shown in FIG. 2 has been heretofore proposed.
Referring now to FIG. 2, which illustrates a prior art receiver, a high frequency signal received by an antenna AT is subjected to HF-amplification and IF-amplification in a tuner TU, and thereafter it is FM-detected in the tuner TU to provide the above-referred synthesized signal. The synthesized signal fed from the tuner TU is applied to a pilot signal amplifier PA, in which the pilot signal (19 KH.sub.z) is amplified. The amplified pilot is frequency-multiplied by a factor of m (normally by a factor of 2) in a first frequency-multiplier Mul.sub.1, and then it is applied to a first switching circuit Sw.sub.1. The output signal of the frequency-multiplier Mul.sub.1 is applied to a phase-shifter PS and a second frequency-multiplier Mul.sub.2. The phase-shifter PS shifts the phase of the output signal of the frequency-multiplier Mul.sub.1 (normally by .pi./2) and feeds the phase-shifted signal to a second switching circuit SW.sub.2. The second frequency-multiplier Mul.sub.2 further multiplies the frequency of the output of the first frequency multiplier Mul.sub.1 to emit at its output a signal having a frequency n times (normally n=4) higher than the frequency of the pilot signal, and it feeds this output signal to a third switching circuit SW.sub.3.
The first, second and third switching circuits SW.sub.1, SW.sub.2 and SW.sub.3 are also applied with a synthesized signal appearing at the output of the tuner TU. Accordingly, in the first switching circuit are demodulated the main channel and the first sub-channel to provide the signals of (LF + LB) and (RF + RB), in the second switching circuit is demodulated the second sub-channel to provide a signal of (LF - LB) + (RF - RB) and an opposite phase signal thereof, and in the third switching circuit is demodulated the third sub-channel to provide a signal of (LF - LB) - (RF - RB) and an opposite phase signal thereof. These demodulated signals are fed to a de-matrix circuit M.sub.x in which each of the LF, LB, RF and RB signals is separately obtained. In FIG. 2, blocks designated by B.A. represent buffer amplifiers. It is to be noted that in the aforementioned construction, the pilot signal amplifier PA is preferably constructed as a phase-locked type amplifier.
In the circuit shown in FIG. 2, the demodulation of the third sub-channel S.sub.3 is carried out through switching with a switching signal of, for example, 76 KH.sub.z to the signal shown in FIG. 1, so that frequency components extending over the double side-band, for example, frequency components 61 KH.sub.z - 91 KH.sub.z, are demodulated. Therefore, in the systems shown in FIG. 1 which use a single side-band as the third sub-channel, unnecessary frequency components existing in the other side-band (in FIGS. 1a and 1c 76-91 KH.sub.z, and in FIGS. 1b and 1d 61-76 KH.sub.z) would appear in the demodulated signal, resulting in noise, thus degrading the S/N ratio of the demodulated signal.